We present a photonic crystal narrow band optical filter,which is made of two pieces of photonic crystals with different photonic band gaps.The principle for the system is to arrange properly the band gaps of the two photonic crystals so that a small narrow pass band appears in the combined photonic band gap region.Our numerical investigations show that this kind of structure is feasible for narrow band optical filtering.As an example,we designed a set of filters at 1 500 nm with a channel interval of 8 nm for ordinary wavelength division multiplexing (WDM) and a set of filters at 1 550 nm with a channel interval of 0.8 nm for dense wavelegnth division multiplexing (DWDM).